Switching circuit

ABSTRACT

In one embodiment, a switching circuit includes a first switch comprising one or more transistors operably coupled in series with a first terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; and a second switch comprising one or more transistors operably coupled in series with a second terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; wherein a source of the one or more transistors of the first switch is operably coupled to a source of the one or more transistors of the second switch.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation in part of U.S. patent application Ser. No. 15/046,585, filed Feb. 18, 2016 (now U.S. Pat. No. 9,525,412), which is a continuation in part of U.S. patent application Ser. No. 14/734,053, filed Jun. 9, 2015 (now U.S. Pat. No. 9,306,533), which claims the benefit of U.S. Provisional Patent Application No. 62/118,552, filed Feb. 20, 2015. U.S. patent application Ser. No. 15/046,585 further claims the benefit of U.S. Provisional Patent Application No. 62/117,728, filed on Feb. 18, 2015. The foregoing references are incorporated herein by reference in their entireties.

BACKGROUND

Semiconductor wafer fabrication can use plasma processing to manufacture semiconductor devices, such as microprocessors, memory chips, and other integrated circuits and devices. Plasma processing involves energizing a gas mixture by introducing RF energy. This gas mixture is typically contained in a vacuum chamber, also called plasma chamber, and the RF energy is introduced through electrodes or other means in the chamber. In a typical plasma process, the RF generator generates power at an RF frequency and this power is transmitted through RF cables to the plasma chamber.

To provide efficient transfer of power from the generator to the plasma chamber, a matching network is used. The purpose of the matching network is to transform the plasma impedance (usually a low value) to a value suitable for the RF generator. In many cases, particularly in wafer fabrication processes, the RF power is transmitted through 50 Ohm coaxial cables and the system impedance of the RF generators is also 50 Ohm. On the other hand, the impedance of the plasma, driven by the RF power, varies and this impedance must be transformed to non-reactive 50 Ohm (i.e., 50+j0) for maximum power transmission. RF matching networks perform this task of continuously transforming the plasma impedance to 50 Ohm for the RF generator. A typical RF matching network includes variable capacitors and a microprocessor-based control circuit to control the capacitance values of the variable capacitors. The value and size of the variable capacitors are influenced by the power handling capability, the frequency of operation, and the impedance range of the plasma chamber.

The predominant variable capacitor in use in RF matching networks is the Vacuum Variable Capacitor (VVC). Electronically Variable Capacitor (EVC) technology, however, is emerging as an alternative, as EVCs can be switched more quickly. Faster switching enables faster matching, which enables faster manufacturing.

High power applications, however, present additional challenges. In low power applications, the use of EVCs is more straightforward because a low voltage matching network can be used. When a matching network is used in a high-power application, however, a high voltage and current matching network is required. In a high voltage matching network, the ability to quickly switch EVCs may be limited by the types of high voltage switches that are used to switch the EVCs themselves. Thus, there exists a need for a high voltage and high current switch that can be used in conjunction with an EVC in an RF matching network for quick switching of high voltages (e.g., 2000 volts or more). Further, beyond semiconductor fabrication and their matching networks, there exists a general need for an improved high voltage and high current switch.

BRIEF SUMMARY

The present disclosure is directed to a switching circuit and methods, systems, and networks for same. In one aspect, a switching circuit includes a first switch coupled to a first switch terminal, the first switch comprising at least one gallium nitride high-electron mobility transistor (GaN HEMT); a second switch coupled in series with the first switch and a second switch terminal, the second switching comprising a GaN HEMT; at least one power source configured to provide power to the first switch and the second switch; wherein the second switch configured to drive the first switch ON and OFF.

In another aspect, a method of controlling a switching circuit includes coupling a first switch to a first switch terminal, the first switch comprising at least one gallium nitride high-electron mobility transistor; coupling a second switch in series with the first switch and a second switch terminal, the second switching comprising a GaN HEMT, wherein the second switch configured to drive the first switch ON and OFF; coupling at least one power source to the first switch and the second switch; driving the first switch ON to thereby drive the first and second switches simultaneously ON; and driving the first switch OFF to thereby drive the first and second switches simultaneously OFF.

In another aspect, a switching circuit includes a first switch comprising one or more transistors operably coupled in series with a first terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; and a second switch comprising one or more transistors operably coupled in series with a second terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; wherein a source of the one or more transistors of the first switch is operably coupled to a source of the one or more transistors of the second switch.

In another aspect, a method of switching a circuit includes providing a first switch comprising one or more transistors operably coupled in series with a first terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; providing a second switch comprising one or more transistors operably coupled in series with a second terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; coupling a source of the one or more transistors of the first switch to a source of the one or more transistors of the second switch; driving the first and second switches simultaneously ON; and driving the first and second switches simultaneously OFF.

In another aspect, a method of fabricating a semiconductor includes placing a substrate in a plasma chamber configured to deposit a material layer onto the substrate or etch a material layer from the substrate; energizing plasma within the plasma chamber by coupling RF power from an RF source into the plasma chamber to perform a deposition or etching; and while energizing the plasma, carrying out an impedance match by an impedance matching network coupled between a load and an RF source, wherein the impedance matching network comprises a first variable component providing a first variable capacitance or inductance; and a second variable component providing a second variable capacitance or inductance; and wherein each of the first variable component and the second variable component has a plurality of switching circuits configured to provide the first variable capacitance or inductance and the second variable capacitance or inductance, respectively, each of the plurality of switching circuits comprising a first switch comprising one or more transistors operably coupled in series with a first terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; and a second switch comprising one or more transistors operably coupled in series with a second terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; wherein a source of the one or more transistors of the first switch is operably coupled to a source of the one or more transistors of the second switch.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description of the exemplary embodiments, will be better understood when read in conjunction with the appended drawings. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown in the following figures:

FIG. 1A is a schematic representation of a first embodiment of an RF impedance matching network.

FIG. 1B is a representation of a first embodiment of a virtual ground.

FIG. 2 is a schematic representation of a second embodiment of an RF impedance matching network.

FIG. 3 is a schematic representation of a third embodiment of an RF impedance matching network.

FIG. 4 is a schematic representation of a fourth embodiment of an RF impedance matching network.

FIG. 5 is a schematic representation of a fifth embodiment of an RF impedance matching network.

FIG. 6 is a schematic representation of a sixth embodiment of an RF impedance matching network.

FIG. 7 is a schematic representation of a seventh embodiment of an RF impedance matching network.

FIG. 8 is a schematic representation of an eighth embodiment of an RF impedance matching network.

FIG. 9 is a schematic representation of a ninth embodiment of an RF impedance matching network.

FIG. 10 is a schematic representation of a tenth embodiment of an RF impedance matching network.

FIG. 11 is a schematic representation of a first switching circuit.

FIG. 12 is a schematic representation of a second switching circuit.

FIG. 13 illustrates parasitic capacitances on a switching circuit.

FIG. 14 is a graph of a switched waveform.

FIG. 15 is a block diagram of a switching circuit.

FIG. 16 is a schematic representation of a third switching circuit.

FIG. 17 is a schematic representation of a fourth switching circuit.

DETAILED DESCRIPTION

The description of illustrative embodiments according to principles of the present invention is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description of embodiments of the invention disclosed herein, where circuits are shown and described, one of skill in the art will recognize that for the sake of clarity, not all desirable or useful peripheral circuits and/or components are shown in the figures or described in the description. Moreover, the features and benefits of the invention are illustrated by reference to the disclosed embodiments. Accordingly, the invention expressly should not be limited to such disclosed embodiments illustrating some possible non-limiting combinations of features that may exist alone or in other combinations of features; the scope of the invention being defined by the claims appended hereto.

As used throughout, ranges are used as shorthand for describing each and every value that is within the range. Any value within the range can be selected as the terminus of the range. In addition, all references cited herein are hereby incorporated by reference in their entireties. In the event of a conflict in a definition in the present disclosure and that of a cited reference, the present disclosure controls.

In the embodiments of an RF impedance matching network disclosed herein, the matching network is configured as a “PI” type matching network. By this configuration, the switching of the variable capacitance components and variable inductance components (sometimes referred to collectively as variable components or reactive components) occurs in the shunt of the matching circuit. Thus, unlike in other matching networks, such as “L” type matching networks, the exemplified matching network switches its reactive components to a ground and not in series. There are no variable inductors or capacitors in the series leg.

In these embodiments, an RF impedance matching network includes an RF input configured to couple to an RF source, the RF source having a fixed RF source impedance; an RF output configured to couple to a load, the load having a variable load impedance; a transformation circuit coupled to the RF input and configured to provide a transformed impedance that is less than the fixed source impedance; and a PI circuit having a first shunt circuit in parallel to the RF input and a second shunt circuit in parallel to the RF input and the RF output. The first shunt circuit includes a first shunt variable component providing a first variable capacitance or inductance; and a first virtual ground coupled to the first shunt variable component and a ground. The second shunt circuit includes a second shunt variable component providing a second variable capacitance or inductance; and a second virtual ground coupled to the second shunt variable component and the ground.

In general, the circuit configurations are tailored to either an inductive or capacitive load impedance of the chamber. Whether one configures a shunt circuit as an inductor or a capacitor will depend on factors such as frequency of operation, power, and the appropriate physical size of the element. For example, smaller inductors will be easier to package and layout, with lower heat dissipation that is easier to control. Specific embodiments will be described in more detail below.

Turning in detail to the drawings, FIG. 1A illustrates an RF impedance matching network 100 according to a first embodiment of the invention. The matching network 100 includes an RF input 160 configured to couple to an RF source 110, and an RF output 170 coupled to a load 120. A sensor 162 is coupled at the RF input 160 between the RF source 110 and the matching network 100. In the exemplified embodiment, the load 120 is a plasma chamber for semiconductor device fabrication. The semiconductor device can be microprocessor, a memory chip, or another type of integrated circuit or device. In other embodiments, the load 120 can be any load of variable impedance that can utilize an RF matching network.

In the exemplified embodiment, the RF impedance matching network 100 serves to help maximize the amount of RF power transferred from the RF source 110 to the plasma chamber 120 by matching the impedance at the RF input 160 to the fixed impedance of the RF source 110. The matching network 100 can consist of a single module within a single housing designed for electrical connection to the RF source 110 and plasma chamber 120. In other embodiments, the components of the matching network 100 can be located in different housings, some components can be outside of the housing, and/or some components can share a housing with a component outside the matching network 100.

As is known in the art, the plasma within a plasma chamber 120 typically undergoes certain fluctuations outside of operational control so that the impedance presented by the plasma chamber 120 is a variable impedance. Since the variable impedance of the plasma chamber 120 cannot be fully controlled, an impedance matching network may be used to create an impedance match between the plasma chamber 120 and the RF source 110.

Moreover, the impedance of the RF source 110 may be fixed at a set value by the design of the particular RF source 110. Although the fixed impedance of an RF source 110 may undergo minor fluctuations during use, due to, for example, temperature or other environmental variations, the impedance of the RF source 110 is still considered a fixed impedance for purposes of impedance matching because the fluctuations do not significantly vary the fixed impedance from the originally set impedance value. Other types of RF sources may be designed so that the impedance of the RF source may be set at the time of, or during, use. The impedance of such types of RF sources is still considered fixed because it may be controlled by a user (or at least controlled by a programmable controller) and the set value of the impedance may be known at any time during operation, thus making the set value effectively a fixed impedance.

The RF source 110 may be an RF generator of a type that is well-known in the art to generate an RF signal at an appropriate frequency and power for the process performed within the plasma chamber 120. The RF source 110 may be electrically connected to the RF input 160 of the RF impedance matching network 100 using a coaxial cable or similar means, which for impedance matching purposes would have the same fixed impedance as the RF source 110.

The plasma chamber 120 can include a first electrode and a second electrode, and in processes that are well known in the art, the first and second electrodes, in conjunction with appropriate control systems (not shown) and the plasma in the plasma chamber 120, enable one or both of deposition of materials onto a substrate and etching of materials from the substrate.

The sensor 162 is configured to monitor the RF signal output from the RF source 110. The sensor 162 can monitor an RF input parameter or multiple RF input parameters at the RF input 160. The sensor 162 can be any sensor configured to detect a parameter at the RF input 160. The input parameter can be any parameter measurable at the RF input 160 sufficient for operating the matching network 100. In the exemplified embodiment, the sensor 162 detects the voltage, current, and phase at the RF input 160 of the matching network 100. The sensor 162 can provide analog information to a control unit 180, where it is converted to a digital form. Based on the RF input parameter detected by the RF input sensor 162, the control unit 180 can determine the variable load impedance. The control unit 180 can further calculate the necessary switching positions on the shunt legs such that the proper inductance and capacitance is provided by the variable components 144, 154 for creating an impedance match. That is, the control unit 180 can determine a first shunt capacitance value for the first shunt variable capacitance component 144 and a second shunt capacitance value for the second shunt variable capacitance component 154 to create an impedance match at the RF input 160. The control unit 180 can then send a control the driver circuit 185 to alter a first shunt variable capacitance of the first shunt variable capacitance component 144; and alter a second shunt variable capacitance of the second shunt variable capacitance component 154 based on a second control signal received from the control unit 180. The match need not be a perfect matching of impedance. For example, an impedance match can result in 10% or less RF power reflected back to the RF source.

Most of the inductive and capacitive components used in the discussed embodiments can be designed on a ceramic substrate or some other material such as Rogers material that can withstand the temperature excursions. Particularly at higher power levels and large currents a capacitive array or/and inductive array may be specifically packaged to satisfy the thermal issues associated with current passing through the shunt elements and series elements at higher power levels. Many of these components will be either air cooled or water cooled, depending on the particular architecture used for a specific load.

Transformation Circuit

The matching network 100 of the exemplified embodiment includes both a PI circuit 135 and a transformation circuit 130. The fixed impedance transformation circuit 130 is located at the front-end of the matching network 100. The transformation circuit 130 is coupled to the RF input 160 and enables the impedance at cross-section A-A looking back toward RF input 160 to be lowered to a value that is less than the real part of the fixed RF source impedance, thus providing a desired lowered impedance at the input of the PI circuit 135 (the PI circuit input 132) that is less than the fixed impedance of the RF source 110. In this embodiment, the RF source impedance is fixed at 50 Ohms, and RF power is transmitted through coaxial cables which also have a fixed impedance of 50 Ohms. In other embodiments, the fixed impedance can be of a different value. In preferred embodiments, the lowered impedance is less than the real part of the load impedance (R_(L)).

The transformation circuit 130 includes an inductor L1 (sometimes referred to as a transformation inductor) and a capacitor C1 (sometimes referred to as a transformation capacitor). In this embodiment, the transformation inductor L1 is in series with the RF input 160, and the transformation capacitor C1 is in parallel to the RF input 160 and coupled to a chassis ground GND. In other embodiments, the configuration is reversed, with the capacitor in series with the RF input 160, and the inductor in parallel to the RF input 160 and coupled to the chassis ground GND. The configuration chosen depends on the remaining PI circuit 135 and the configuration that prevents the DC component of the load 120 returning to the RF source 110. The transformation circuit 130 is connected to a chassis ground GND (not the virtual grounds, which will be discussed below). In other embodiments, the chassis ground GND can be another type of ground.

In the exemplified embodiment, the transformation circuit 130 is configured to lower the impedance from 50 Ohms at the RF input 160 to approximately 15 Ohms at the PI circuit input 132 (the input of the PI circuit 135), with no imaginary part (or minimal imaginary part). Thus, for example, the output impedance (Z_(o)) of the transformation circuit 130 is approximately 15 Ohms+j0. The PI circuit 135 of the matching network 100 is designed for this reduced input impedance, referred to as Z_(in) in FIGS. 1A-10. The interface between the output of the transformation circuit 130 and the PI circuit 135 is designated by line A-A.

The transformation circuit 130 can lower the voltage stresses on the matching network 100. Thus, high voltage stress on switches S11 to S1N and S21 to S2N will be lowered. Such lowered stress enhances the functioning of switches such as RF FET switches, PIN diodes, and insulated-gate bipolar transistors (IGBTs).

This lowered stress can be better understood by the following examples. In the first example, there is no transformation circuit. A 5,000 Watts RF generator (RF source) has a 50 Ohms output impedance (R_(source)) and a frequency of 27.12 MHz that is provided at the RF input 160 of the matching network 100. The matching network 100 is perfectly tuned to 50 Ohms at its input and has no reactive components present. The voltage (V=√{square root over (PR)}) therefore will be √{square root over ((5000 W)(50 Ohms))}) or 500 V rms. The current (I=V/R) will be 500 V rms/50 Ohms, or 10 A rms.

In the second example, a transformation circuit lowers the impedance provided at the input 132 of the PI circuit 135 to 15 Ohms. The voltage (V=√{square root over (PR)}) will now be √{square root over ((5000 W)(15 Ohms))}) or 273.9 V rms, and the current (I=V/R) will be 273.9 V rms/15 Ohms, or 18.3 A rms. Thus, the current increases by a factor of 1.83, while the voltage decreases by a factor of 1.83. The active components of the PI circuit 135 more easily handle current than voltage. Thus, the transformation circuit's reduction of voltage causes less stress on the active components. A designer can choose for his convenience appropriate impedance reduction factor. That reduction factor depends on chamber load impedance and its real part (Z_(L)=R_(L)+/−jX_(L)). In preferred embodiments, the reduced input impedance Z_(in) is less than the real part of the load impedance (R_(L)).

PI Circuit

The PI circuit 135 of FIG. 1A includes a series capacitor C3 in series with the RF input 160 and coupled between the transformation inductor L1 and the RF output 170. The series capacitor C3 can decouple the DC component to help prevent the DC component of the load 120 from returning to the RF source 110. The PI circuit 135 further includes a series inductor L2 in series with the RF input 160 and coupled between the series capacitor C3 and the RF output 170. The PI circuit 135 further includes a first shunt circuit 140 in parallel to the RF input 160 and a second shunt circuit 150 parallel to the RF input 160.

The first shunt circuit 140 includes a first shunt capacitor C_(DC) and a first shunt padding inductor L3 _(P). The first shunt capacitor C_(DC) can decouple the DC component of the plasma coming back toward the switches S11, S12, S1N, such that the switches are not burdened with large components of the DC voltage. The first shunt circuit 140 further includes a first shunt variable inductive component 144 comprising (a) a plurality of first shunt inductors L31, L32, L32 coupled in parallel to the first shunt padding inductor L3 _(P), and (b) a plurality of first shunt switches S11, S12, S1N coupled to the plurality of first shunt inductors L31, L32, L32 and configured to connect and disconnect each of the plurality of first shunt inductors L31, L32, L32 to a first virtual ground 142. Also included is a first shunt ground capacitor C1 _(gnd) coupled between the second virtual ground and the ground GND. The first shunt ground capacitor C1 _(gnd) and a first shunt ground inductance inherent to the first virtual ground 142 can resonate in series to provide the lowest possible impedance from the virtual ground to the ground GND. The first shunt circuit 140 further includes a first shunt galvanic return resistor R1 _(G) coupled between the first virtual ground 142 and the ground GND. The first shunt ground capacitor C1 _(gnd) and the first shunt galvanic return resistor R1 _(G) are coupled in parallel.

The second shunt circuit 150 includes a second shunt variable capacitance component 154 comprising (a) a plurality of second shunt capacitors C21, C22, C2N coupled in parallel, and (b) a plurality of second shunt switches S21, S22, S2N coupled to the plurality of second shunt capacitors C21, C22, C2N and configured to connect and disconnect each of the plurality of second shunt capacitors C21, C22, C2N to a second virtual ground 152. The circuit further includes a padding capacitor C2 _(P) coupled in parallel to at least one of the plurality of second shunt capacitors C21, C22, C2N, the padding capacitor C2 _(P) coupled to the second virtual ground 152. The circuit further includes a second shunt ground capacitor C2 _(gnd) coupled between the second virtual ground 152 and the ground GND, and a second shunt galvanic return resistor R2 _(G) coupled between the second virtual ground 152 and the ground GND. The second shunt ground capacitor C2 _(gnd) and the first shunt galvanic return resistor R2 _(G) are coupled in parallel. The capacitors, inductors, and resistors discussed herein can refer to any components of suitable design to provide capacitance, inductance, and resistance, respectively. In preferred embodiments, the matching network is designed such that, for a particular load, there is minimal possible variation of capacitors and inductors, thereby allowing the fewest possible switches. Padding capacitors and padding inductors are capacitors and inductors that do not need to be switched.

The variable inductance components discussed herein can refer to a plurality of shunt inductors and coupled switches, and is sometimes referred to as an inductive array or simply a variable inductor. Similarly, the variable capacitance components discussed herein can refer to a plurality of shunt capacitors and coupled switches, and is sometimes referred to as a capacitive array or simply as a variable capacitor. A variable inductance component can refer to any structure capable of providing a variable inductance, and a variable capacitance component can refer to any structure capable of providing a variable capacitance. For example, the variable capacitance component can be an electronic variable capacitor (EVC), as described in U.S. Pat. No. 7,251,121. By these components, the capacitances and inductances provided in the shunt legs can be controlled such that the combined impedances of the RF impedance matching network 100 and the plasma chamber match, or at least substantially match, the fixed impedance of the RF source 110. A first shunt variable inductance component 144 and a second shunt variable capacitance component 154 are shown in FIG. 1A.

Virtual Ground

As discussed above, the switches are not connected directly to chassis ground GND but to a virtual ground 142, 152. FIG. 1B shows an embodiment of the first virtual ground 142. The same or a similar structure can be used for virtual ground 152 and the other virtual grounds disclosed herein. In the exemplified embodiment of FIG. 1B, the virtual ground 142 is an aluminum plate with embedded tubes 147. The virtual ground 142 is water cooled, with water (H₂O) flowing in and out of the embedded tubes 147. The virtual ground can include thermally conductive isolation pads or paste between the virtual ground surface 145 and the chassis GND to conduct heat and to separate the virtual ground from the chassis ground GND. The pad 143 is not electrically conductive. In other embodiments, the virtual ground 142, 152 can be any electrically conductive component that is physically separated from a ground. For example, the virtual ground can be arranged vertically or include fins. Further, the virtual ground can include other cooling means. For example, at low power applications, the virtual ground can be air cooled. Standard heat sinking methods can be applied.

In the exemplified embodiment of FIG. 1B, nine switches S11-S19 are mounted on the surface 145 of the virtual ground 142. The nine switches S11-S19 correspond with switches S11, S12, and S1N of FIG. 1A, since the “N” can refer to any total number of switches (or any total number of inductors or capacitors as “N” is used elsewhere in the drawings). In other embodiments, more or less switches can be used, depending on the required accuracy of the variable inductance (or variable capacitance in other embodiments).

Switchable and padding components L31-L39 and L3 _(P) (corresponding to L31, L32, L3N, L3 _(P) of FIG. 1A) can also be mounted on the surface 145 of the first virtual ground 142, as shown in FIG. 1B. The exemplified embodiment uses nine switchable inductors, though, as with the switches discussed above, any number of switchable inductors (or switchable capacitors depending on the embodiment) can be used. Further, the other virtual grounds disclosed herein can be configured similarly. Thus, the switchable and padding components C21, C22, C2N, C2 _(P) of the second shunt circuit 150 can be mounted on a surface of the second virtual ground 152 in a manner similar to the surface 145 of virtual ground 142 shown in FIG. 1B. Further, similar virtual grounds can be used for the matching network embodiments shown in FIGS. 2-10. For example, virtual ground 242 of FIG. 2 can use a virtual ground configuration similar to that shown in FIG. 1B, but where capacitors C11, C12, C1N, C1 _(P) are mounted on the surface of the virtual ground 242 instead of inductors L31-L39 and L3 _(P).

As shown in FIG. 1A, the virtual ground 142, 152 can be connected to a common RF chassis ground GND via a ground capacitor C1 _(gnd), C2 _(gnd). The virtual ground 142, 152 has an inherent inductance (L_(gnd)) that is generally small. This inherent inductance can connect in series with the ground capacitor to create a series resonant circuit. The series resonant circuit acts as a short at the frequency of operation. There is also a galvanic return resistor designated as R1 _(G) or R2 _(G) that has a much larger value than the series resonance impedance of L_(gnd) and C_(gnd). Thus, the galvanic connection does not pass the main RF current.

By using a virtual ground for each shunt circuit of variable components, each branch return of RF-switched current can go to one point ground. Since the shunt branch RF current can be very large, they will be much easier to control and to stream them away from, for example, FET gate driving low voltage circuitry. Further, noise immunity and RF interference within the matching network will be much easier to control.

By the virtual ground, the switches in a shunt circuit can be connected to a platform from which heat can be better controlled and extracted if needed. The RF currents flowing from the tuning capacitors or inductors (e.g., L31, S21) will always flow into the virtual ground 142. The virtual ground 142 can also reduce the coupling capacitance from the back side of the switches and their respective FETs to a ground. Each variable capacitive or inductive component 144, 154 can have a separate virtual ground 142, 152 to further reduce the capacitive cross talk among the switches. The separation of switched currents in the virtual ground can help reduce the jitter noise in the switches as well as cross talk among the switches. These currents can be very difficult to control at very high RF power levels. The virtual ground can ease the design of high power switches.

After full layout of the circuit, one can determine the inductance of the ground connections. A capacitance can be determined for some very small impedance from virtual ground to the chassis ground GND by calculating the needed capacitance C_(gnd). In a preferred embodiment, the capacitor C_(gnd) has a very low ESR (equivalent series resistance) and should withstand several kilovolts of voltage breakdown in case of overvoltage occurrence on the RF switches. Choosing the proper components can prevent the network from being under high voltage stress. The embodiment shown in FIG. 1A is useful for an inductive load chamber and powers below 5 kW.

In the matching network 100 of FIG. 1A and certain other embodiments discussed hereafter, a bypass capacitor C_(DC) (or C4) forms part of the shunt circuit 140 for the purpose of decoupling the DC voltage that may be reflected from the chamber load 120. This capacitance is not part of the matching network 100 in that this capacitance is not used for matching purposes. The capacitor C_(DC) can sustain high voltage swings coming back from the load 120 and can pass very large RF currents. For instance, at RF power P=5 kW, these currents may be in the order of 100 A rms. The bypass capacitor C_(DC) can be in series with the variable shunt inductance and can prevent the DC voltage returning from the chamber 120 from reaching the RF switches.

In the exemplified embodiment, the switches use field effect transistors (FETs). In other embodiments, the switches use PIN/NIP diodes, a Micro Electro Mechanical (MEM) switch, a solid state relay, bipolar transistors, insulated-gate bipolar transistors (IGBTs), and the like. In the exemplified embodiment, each switch turns a capacitor or inductor ON or OFF to control the overall capacitance or inductance provided by the variable components 144, 154, thus enabling the matching network 100 to provide variable capacitance and variable inductance in the shunt legs. In alternative embodiments, a switch can be configured to control more than one capacitor or inductor.

The matching network 100 includes one or more RF switch driver circuits 185. The driver circuits 185 are configured to switch the plurality of switches S11, S12, S1N, S21, S22, S2N. Each of the driver circuits 185 can include several discrete driving circuits, with each discrete driving circuit configured to switch one of the switches.

The matching network 100 further includes a control unit 180. The control unit 180 is the brains of the RF impedance matching network 100 as it receives multiple inputs from sources such as the sensor 162 and the driver circuits 185 and makes calculations necessary to determine changes to the variable capacitance and inductance components 144, 154, and delivers commands to these components 144, 154 to create the impedance match. The control unit 180 can be of the type that is commonly used in semiconductor fabrication processes, and therefore known to those of skill in the art.

The control unit 180 can be configured with an appropriate processor and/or signal generating circuitry to provide an input signal for controlling the driver circuits 185. The control unit 180 of the matching network 100 can be a standard DSP- and FPGA-based architecture. The control unit 180 can house several other circuits, including an overvoltage conditioning circuit 182 for switching off all the active switches in the case of overvoltage at the output of the match. The overvoltage circuit 182 can indicate to the control board when to go into the shutdown condition.

In the exemplified embodiment, the control unit 180 includes a processor. The processor may be any type of properly programmed processing device, such as a computer or microprocessor, configured for executing computer program instructions (e.g. code). The processor may be embodied in computer and/or server hardware of any suitable type (e.g. desktop, laptop, notebook, tablets, cellular phones, etc.) and may include all the usual ancillary components necessary to form a functional data processing device including without limitation a bus, software and data storage such as volatile and non-volatile memory, input/output devices, graphical user interfaces (GUIs), removable data storage, and wired and/or wireless communication interface devices including Wi-Fi, Bluetooth, LAN, etc. The processor of the exemplified embodiment is configured with specific algorithms to enable matching network 100 to perform the functions described herein.

A power supply (not shown) can be connected to the driver circuits 185, control unit 180, and sensor 162 to provide operational power, at the designed currents and voltages, to each of these components.

The inductive and capacitive shunt designs in the PI configuration enable low voltage stresses on the variable components. High voltage stress is particularly hard on active FET switches that must switch large potentials as well as large currents at the power levels on the order of 5 kW. Since the disclosed embodiments do not switch any series components in these PI configurations, they are fixed in this matching network 100, and therefore there are lower voltages on the shunt capacitive or inductive components. This will be shown later in a tabular form.

At lower frequencies the inductors may be discrete since they will have inherently larger values. At higher frequencies such as 13.56 MHz, 27.12 MHz, 40.68 MHz, and 60 MHz, the inductors can be made by a method called spiral inductors and printed on a ceramic substrate.

FIG. 2 is a schematic representation of a second embodiment of an RF impedance matching network 200. As will be described, this embodiment uses variable capacitance components 244, 254 in both shunt legs. As in FIG. 1A, the matching network 200 includes an RF input 260 configured to couple to an RF source 210 and an RF output 170 configured to couple to a load 220, a transformation circuit 230, and a PI circuit 235.

The transformation circuit 230 is again coupled to the RF input 260 and configured to provide a transformed impedance that is less than the fixed source impedance. The transformation capacitor C1, however, is in series with the RF input 260, and the transformation inductor L1 is in parallel to the RF input and coupled to the chassis ground GND.

The first shunt circuit 240 is in parallel to the RF input 260. The circuit 240 includes a first shunt variable capacitance component 244 comprising (a) a plurality of first shunt capacitors C11, C12, C1N coupled in parallel, and (b) a plurality of first shunt switches S11, S1, S13 coupled to the plurality of first shunt capacitors C11, C12, C1N and configured to connect and disconnect each of the plurality of first shunt capacitors C11, C12, C1N to a first virtual ground 242. The first shunt circuit 240 further includes a padding capacitor C1 _(P) (“first shunt padding capacitor”) coupled in parallel to at least one of the plurality of first shunt capacitors C11, C12, C1N, the first shunt padding capacitor C1 _(P) coupled to the first virtual ground 242; a capacitor C1 _(gnd) (“first shunt ground capacitor”) coupled between the first virtual ground 242 and the ground GND; and a resistor R1 _(G) (“first shunt galvanic return resistor”) coupled between the first virtual ground 242 and the ground GND.

The second shunt circuit 250 is also in parallel to the RF input 260. Similar to the first shunt circuit 240, the second shunt circuit 250 includes a second shunt variable capacitance component 254 comprising (a) a plurality of second shunt capacitors C21, C22, C2N coupled in parallel, and (b) a plurality of second shunt switches S21, S22, S2N coupled to the plurality of second shunt capacitors C21, C22, C2N and configured to connect and disconnect each of the plurality of second shunt capacitors C21, C22, C2N to a second virtual ground 252. The second shunt circuit 250 further includes a padding capacitor C2 _(P) (“second shunt padding capacitor”) coupled in parallel to at least one of the plurality of second shunt capacitors C21, C22, C2N, the second shunt padding capacitor C2 _(P) coupled to the second virtual ground 252; a capacitor C2 _(gnd) (“second shunt ground capacitor”) coupled between the second virtual ground 252 and the ground GND; and a resistor R2 _(G) (“second shunt galvanic return resistor”) coupled between the second virtual ground 252 and the ground GND.

The matching network 200 further includes a series inductor L2 in series with the RF input 260 and coupled between the transformation capacitor C1 and the RF output 270. The embodiment of the matching network 200 shown in FIG. 2 is useful for inductive and capacitive loads. Since it has only one inductor in the series leg, it is efficient. It is useful for high power applications that are less than 10 kW.

FIG. 3 is a schematic representation of a third embodiment of an RF impedance matching network 300. In this embodiment, variable inductors 344, 354 are used in both shunt legs. As in the previous figures, the matching network 300 includes an RF input 360 configured to couple to an RF source 310 and an RF output 370 configured to couple to a load 320, a transformation circuit 330, and a PI circuit 335.

Similar to the embodiment shown in FIG. 1A, the transformation inductor L1 is in series with the RF input 360, and the transformation capacitor C1 is in parallel to the RF input 360 and coupled to a chassis ground GND. Also similar to FIG. 1A, the first shunt circuit 340 includes a first shunt padding inductor L3 _(P); a plurality of first shunt inductors L31, L32, L3N coupled in parallel to the first shunt padding inductor L3 _(P); a plurality of first shunt switches S11, S12, S1N coupled to the plurality of first shunt inductors L31, L32, L3N and configured to connect and disconnect each of the plurality of first shunt inductors L31, L32, L3N to a first virtual ground 342; a first shunt ground capacitor C1 _(gnd) coupled between the second virtual ground 352 and the ground GND; and a first shunt galvanic return resistor R1 _(G) coupled between the first virtual ground 342 and the ground GND. The second shunt circuit 350 is configured similar to the first shunt circuit 340. The matching network 300 further includes a first series capacitor and a second series capacitor coupled in series between the RF input 360 and the RF output 370.

The embodiment of the matching network 300 shown in FIG. 3 has only one inductor in the series leg and therefore is more efficient. It is useful for high power applications up to more than 5 kW. It is useful for inductive loads.

FIG. 4 is a schematic representation of a fourth embodiment of an RF impedance matching network 400. This embodiment is similar to the embodiment shown in FIG. 3, however, the first series capacitor C2 is coupled between the RF input 460 and the first shunt circuit 440, the second series capacitor C3 is coupled between the first shunt circuit 440 and the second shunt circuit 450, and a single capacitor C4 is coupled in series with the padding inductor L4 _(P) in the second shunt circuit 450. The embodiment of the matching network 400 shown in FIG. 4 has only one inductor in the series leg and therefore is useful in higher power matching designs, up to approximately 10 kW. The series capacitor C3 is moved away from high current in the load loop, which improves efficiency and therefore usefulness for high RF power matching.

FIG. 5 is a schematic representation of a fifth embodiment of an RF impedance matching network 500. This embodiment is similar to the embodiment shown in FIG. 1A, however, the series inductor L2 is replaced with a series capacitor C3, a series inductor L4 is added, and single capacitor C_(DC) in the first shunt circuit 140 of FIG. 1A is eliminated. The embodiment of the matching network 500 shown in FIG. 5 is useful at lower powers up to 5 kW.

FIG. 6 is a schematic representation of a sixth embodiment of an RF impedance matching network 600. This embodiment is similar to the embodiment shown in FIG. 1A, however, the inductor L1 and capacitor C1 of the transformation circuit 630 are reversed, the series capacitor C3 of FIG. 1A is eliminated, and the positioning of the first and second shunt circuit 650 s is reversed, such that the first shunt circuit 640 has the plurality of switching capacitors C21, C22, C2N, and the second shunt circuit 650 has the plurality of switching inductors L41, L42, L4N. The embodiment of the matching network 600 shown in FIG. 6 is useful for medium-to-high powers and inductive loads. Since it has only one inductor in the series leg, it can provide good efficiency.

FIG. 7 is a schematic representation of a seventh embodiment of an RF impedance matching network 700. This embodiment is similar to the embodiment shown in FIG. 2, however, a series inductor L3 is coupled between the transformation capacitor C1 and the first shunt circuit 740. The embodiment of the matching network 700 shown in FIG. 7 has two inductors in the series leg, and therefore is used for lower powers below 5 kW.

FIG. 8 is a schematic representation of an eighth embodiment of an RF impedance matching network 800. This embodiment is similar to the embodiment shown in FIG. 6, however, the inductor L1 and capacitor C1 of the transformation circuit 830 are reversed, a series inductor L2 is added between the transformation inductor L1 and the first shunt circuit 840, and the series inductor L2 between the first and second shunt circuits 840, 850 is replaced with a series capacitor C3. The embodiment of the matching network 800 shown in FIG. 8 is useful for capacitive chamber loads. There are two inductors in the series leg and it is useful for applications below 5 kW.

FIG. 9 is a schematic representation of a ninth embodiment of an RF impedance matching network 900. This embodiment is similar to the embodiment shown in FIG. 2, however, an additional series inductor L3 is coupled between the second shunt circuit 950 and the RF output 970. The embodiment of the matching network 900 shown in FIG. 9 is useful for capacitive chamber loads. There are two inductors in the series leg and therefore more losses. It is useful for applications below 5 kW.

FIG. 10 is a schematic representation of a tenth embodiment of an RF impedance matching network 1000. This embodiment is similar to the embodiment shown in FIG. 8, however, the series inductor L3 is coupled between the first shunt circuit 1040 and the second shunt circuit 1050, and the series capacitor C3 is coupled between the second shunt circuit 1050 and the RF output 1070. The embodiment of the matching network 1000 shown in FIG. 10 is useful for inductive chamber loads and for medium-to-low RF power applications. It is noted that, where components in one embodiment are similarly situated to components in another embodiment, the function and characteristics of those components can be similar, though a detailed discussion of those components is not repeated for that particular embodiment.

In Table 1, simulated results for typical matching conditions are shown for the matching network 100 shown in FIG. 1A. The impedance load conditions are assumed as typical chamber impedances. Values for series coupling capacitor C3 (2 nF) and series inductance L2 (8 uH) were specifically chosen to satisfy the circuit equations under the matching conditions. R_(L) is representative of the real part of the load impedance. P_(out) is the delivered RF power to the load resistor R_(L). The example components were chosen for delivering 5 kW of RF power to a plasma chamber. The simulations show that the matching network can satisfy all the load impedance conditions for a typical process.

TABLE 1 Matching Network of FIG. 1A Simulation Results. L31, L32, L2 C21, C22, R_(L) X_(L) C3 L3N (var.) (fixed) C2N (var.) I_(C3) I_(L2) I_(L) V_(L) P_(out) Ω Ω nF uH uH pF A, rms A, rms A, rms kV, rms kW 1 31 2 2.5 8 3,080 9.8 15.3 66.4 1.6 5 2 38 2 3.1 8 2,640 11.3 13.3 45 1.4 5 5 39 2 5 8 2,400 9.9 8.0 30.5 0.8 5.8 10 39 2 5 8 2,300 13.3 6.8 20 0.7 5.1

Software was used to determine the circuit components values for the variable shunt elements L3, C2 using the assigned load impedance values. The software then calculated the currents and voltages. As is shown, the matching network can be designed with reasonable values for the capacitors and inductors. The simulation was carried out at the frequency f=2 MHz, and therefore the components values reflect that frequency. The assumed power was 5 kW. One could choose other value for the variable shunt elements L3, C2 to attempt to have even smaller currents and voltages in the variable shunt elements L3, C2.

Switching Circuits

FIGS. 11-17 below discuss switching circuits that can be utilized in the above matching networks. For example, each of switches S11 to S1N and S21 to S2N in FIG. 2 can utilize one of the switching circuits discussed below to switch capacitors C11 to C1N and C21 to C2N, respectively, thus enabling electronically variable capacitance. Similarly, the switching circuits can switch the inductors L31 to L3N of FIG. 1 to enable variable inductance. Accordingly, switches S11 to S19 of FIG. 1B can utilize one of the switching circuits discussed below.

The switching circuits discussed below can also be used outside of RF matching networks, and outside the realm of high frequency switching. For example, certain switching circuits may be used as a form of high voltage relay at a low frequency. The disclosed switching circuits can also be used in equipment such as magnetic resonance circuits, medical equipment circuits (e.g., ultrasound imaging or x-ray equipment). Another possible application is in telecommunications equipment where the antenna impedance should be matched to the transmitter output impedance. In such cases as transmitters, there are many applications where the carrier frequency is switched under so-called hot switching conditions. These switches are also used in handheld transmitters for military use, and in many other military applications using various transmitters and receivers.

Referring now to FIG. 11, a first switching circuit 1100 is shown. The first switching circuit 1100 comprises a passive switch 1110 coupled to a first switch terminal A. The first switching circuit 1100 further comprises a driving switch M1 coupled in series with the passive switch 1110 and a second switch terminal B, the driving switch M1 configured to turn the passive 1110 switch on and off. The first switching circuit 1100 further comprises a power source 1160 configured to provide power to the passive switch 1110 and the driving switch M1. The first switching circuit 1100 further comprises a monitoring circuit 1150 configured to (1) receive an indication that a switching circuit voltage exceeds a predetermined amount and, in response, reduce the power provided to the driving switch M1; and (2) receive an indication that a switching circuit current exceeds a predetermined amount and, in response, reduce the power provided to the driving switch M1. In other embodiments, the monitoring circuit 1150 can be configured to perform operation (1) and not operation (2), or operation (2) and not operation (1). These components and operation will be described further below.

In the exemplified embodiment, the power source 1160 is a switched mode power supply (SMPS). Further, the exemplified switching circuit includes a gate driver 1102 operably coupled to (a) the power source 1160, (b) a gate of the passive switch 1110, and (c) a gate of the driving switch M1. In other embodiments, other power sources, drivers, and/or amplifiers can be used as required by the components of the specific switching circuit.

In the exemplified embodiment, the passive switch 1110 comprises a plurality of switches connected in series. Specifically, the passive switch 1110 comprises two high voltage and high current junction gate field-effect transistors (JFETs) J1, J2. In other embodiments, the passive switch can comprise any number of switches, and those switches can be of various types. The exemplified JFETs of the passive switch 1110 are connected in series to increase the voltage operation of the switching circuit 1100. The passive switch 1110 can also comprise external body diodes.

The first switching circuit 1100 further comprises a driving switch M1 coupled in series with the passive switch 1110 and a second switch terminal B. The driving switch M1 can be configured to turn the passive switch 1110 on and off. In the exemplified embodiment, the driving switch M1 comprises a high voltage and high current metal-oxide-semiconductor field-effect transistor (MOSFET). In other embodiments, other switches can be used, such as a BJT, an IGBT, a GaN device, and even a relay (e.g., in special applications at low frequency and high currents). Connecting the driving switch M1 and passive switch 1110 in series allows the circuit to use an industry standard gate driver integrated circuit (IC) with the driving switch M1.

The switching circuit 1100 can be configured such that JFETS J1, J2 are always turned on, and thereby the JFETS J1, J2 can provide high voltage isolation 1170. Resistors R3 and R4 (which can have large values, e.g., 10 Megaohms) can connect the gate and drain of the JFETs J1, J2 to the same potential and can force the JFETS to the on condition. The drain voltages starting on terminal A in FIG. 11 can distribute equally on all three transistors M1, J1, J2. In other embodiments, the switching circuit can include more than two JFETS in series, but the packaging of these devices must be such that the high voltage would not break down on the junctions. A separate circuit that allows equal voltage distribution on the switches is omitted here, as such circuits (e.g., a snubber circuit) are well known. These switches are typically heat sunk either on an air heat sink or water-cooled heat sink. All these switching arrangements can also use GaN devices since in the ON state they conduct current in both directions. Driving GaN gates, however, requires slightly different gate drivers.

The first switching circuit 1100 can also include an optocoupler 1106 configured to electrically isolate the gate driver IC 1102 from the monitoring circuit 1150 (discussed below). The optocoupler 1106 can drive the gate driver IC 1102 on and off, thus also driving the first switching circuit 1100 on and off. Supply line +V can be around 10-15 VDC and can be supplied by a switched-mode power supply (SMPS). DC isolation on the SMPS is expected to be greater than 4 kV for this application, thus enabling the switching circuit 1100 to be used in a high voltage RF application. In the exemplified embodiment, the low voltage monitoring circuit 1150 is isolated from the drains and high voltage switching voltage by at least 4 kV peak, if not more in other applications. For higher power applications on this switching circuit, the voltage separation can be raised even higher. The gate driver can have the floating SMPS associated with it for that purpose.

The exemplified optocoupler 1106 and gate driver IC 1102 are not connected to chassis ground GND but to a virtual ground. The main RF ground (terminal B) is connected to the virtual ground. The virtual ground can be designed similarly to the virtual ground shown in FIG. 1B, which is an aluminum plate with embedded tubes for water cooling. Mounted on the virtual ground can be variable capacitors or inductors and a switching circuit for each. Thus, components of the switching circuit 1100 (e.g., power FETs) can be mounted on the virtual ground. As discussed above, the virtual ground can be connected via a series network directly to the chassis ground.

The exemplified virtual ground is floating, and it has an isolation of the systems grounds as large as the transformers and the optocouplers will allow. For that reason, two or more such system switches can be connected in parallel to increase the current switching capabilities.

The first switching circuit 1100 can be considered “smart” or “self-healing” as it has two automatic shut-off conditions—one for excessive voltage and one for excessive current. Regarding excessive voltage, in the exemplified embodiment, the switching circuit 1100 can be switched off if a voltage on terminal A exceeds a predetermined value. For example, the predetermined value can be 4,000 V.

An excessive voltage indication can be provided by a voltage sensing circuit 1130, the voltage sensing circuit 1130 comprising (a) a first transformer T1 operably coupled to the second terminal B and (b) a first diode D1 operably coupled to the monitoring circuit 1150. The switching circuit 1100 can be designed such that, if the voltage at terminal A is 4,000 V, then the voltage at diode D1 output (voltage sensing circuit output) is 4 VDC. The voltage at the diode D1 output can provide evidence of excessive voltage at terminal A or elsewhere in the circuit.

Diode D1 and the optocoupler 1106 can be connected to a monitoring circuit 1150 for monitoring the switching circuit 1100 for excessive voltage or current. In the exemplified embodiment, the monitoring circuit 1150 can be configured to (1) receive an indication that a switching circuit voltage exceeds a predetermined amount (excessive voltage indication) and, in response, reduce the power provided to the driving switch M1; and (2) receive an indication that a switching circuit current exceeds a predetermined amount (excessive current indication) and, in response, reduce the power provided to the driving switch M1.

The monitoring circuit 1150 can include low-level logic components to determine whether a shut-off condition is present. For example, regarding excessive voltage, a first comparator 1151 can be used to receive the excessive voltage indication. The first comparator 1151 can receive a threshold voltage V_(ref) on a first terminal and can be set to normally provide an output a logical value “1” (e.g., +4 V). The first comparator 1151 can also receive a signal from the voltage sensing circuit 1130. When the voltage at the voltage sensing circuit output (the monitor voltage) exceeds the V_(ref), the comparator can change its condition to logical “0” (e.g., 0 V), thus causing the shut off condition on gate 1153 for the control signal 1154.

The second shut off condition of the switching circuit 1100 can be based on current. The switching circuit 1100 can include a current sensing circuit 1140. The current sensing circuit 1140 can determine a current passing from a first switch terminal A to a second switch terminal B of the first switching circuit when the switching circuit is turned on by the control signal 1154. That current can be determined by the RF impedance load that the first switching circuit 1100 is switching on and off. This current is similar to load current I_(L) in FIGS. 1A and 3-10. The current sensing circuit 1140 can include (a) a second transformer T2 operably coupled to the second terminal B and (b) a second diode D2 operably coupled to the monitoring circuit 1150. The current sensing circuit 1140 can further include sense resistor R_(S). Sense resistor R_(S) is shown adjacent to terminal B in FIG. 11, and its surrounding components are shown in more detail below as part of sensing circuit 1140. Sense resistor R_(S) can provide a low resistance (e.g., 10-50 mOhms). The circuitry that follows the sense resistor R_(S) can function like a high-speed operational amplifier with sufficient gain bandwidth product to replicate the RF current waveform. The gain in the operational amplifier chain must be such that the switching circuit 1100 can detect an appreciable RF voltage. In some cases, such as at lower frequencies and high currents, the switching circuit 1100 can use a current transformer in the source of driving switch M1 instead of a sense resistor. If the inductance of the current transformer is sufficiently small at the frequency of operation, it can be substituted for the R_(S) sense resistor.

The current in the source of the driving switch M1 can thus be detected and received via the source resistor R_(S). The processing of the current waveform is received by the source resistor R_(S) and the operational amplifier circuitry that follows the detected waveform. The signature of the current in the source of the driving switch M1 can be used to see its amplitude. This current amplitude is detected and can be viewed on an oscilloscope to determine the shape and the frequency spectrum, if necessary. Further on, after the output of the operational amplifier A1, there is diode D2. The output of that diode D2 (used as a detector) is a DC voltage proportional to the RMS value of the source current in the driving switch M1. This detected DC voltage waveform can be sent to the second comparator 1152, with a reference voltage V_(ref) on the other terminal of the second comparator 1152. When the detected voltage, which is now proportional to the current in the source of the driving switch M1, is larger than a predetermined value, this can be considered an excessive current indication. In this case, the reference voltage can trip the second comparator 1152 output from its normally high value (logic value “F”) to a low value (logic value “0”). The monitoring circuit 1150 can thus detect that the current in the driving switch M1 has exceeded the predetermined value.

The monitoring circuit 1150 can include an AND logic gate 1153 to switch off the gate driver IC 1102 when a shut off condition has occurred. In the exemplified embodiment, a three-input wide AND logic gate 1153 is utilized. The detector diodes D1, D2, as described above, rectify the RF waveforms of detected RF voltage and current and covert them to DC so that the signals can be used at the comparator terminals. The comparators 1151, 1152 receive signals from the detector diodes D1, D2. The comparer outputs can be logic signals that are received by the AND logic gate 1153. The AND logic gate 1153 can shut off the incoming control signal 1154 if a high voltage or high current condition is indicated. In the exemplified embodiment, when a high voltage condition is indicated the first comparator output is a logic value 0, and when a high voltage condition is indicated the second comparator output is a logic value 0. If the AND logic gate 1153 receives a logic value 0 from either comparator 1151, 1152, the AND logic gate 1153 can shut down the control signal 1154 and thereby switch off the gate driver IC 1102. In so doing, the monitoring circuit 1150 can ensure safe voltage and current operation values. The set of safe operating values can be set by the appropriate reference voltage values on the comparators 1151 and 1152.

The foregoing circuitry can further signal an LED light to tell an operator that the switching circuit 1100 has exceeded the prescribed voltage or current value. The LED light can stay on as long as there is an excessive voltage or current condition. In other embodiments, the switching circuit 1100 can utilize methods other than those described above for responding to excessive voltage or excessive current conditions on the passive switch 1110 and the driving switch M1.

There are several advantages to the described self-healing switch. Other solutions for shutting off such switches could include the use of control board algorithms. But such an approach is usually too slow to shut off the driving switch M1 off when the voltage becomes excessive. The exemplified analog hardware implementation is much faster. The voltage sensing circuit 1130 is designed such that its frequency bandwidth can be large enough to cover at least the third harmonic of the RF frequency that is being switched. For example, if the switching circuit 1100 is used in the standard ISM frequency of 13.56 MHz, the bandwidth of the voltage sensing circuit 1130 can be larger than 40.68 MHz. This large frequency bandwidth also enables one to see the shape of the switched waveform during the switching conditions. That shape can be observed at terminal A2 before the detector diode D1.

The exemplified switching circuit 1100 can prevent a catastrophic switch failure in the case of high voltage transient returning back from the vacuum chamber via the matching network and on the RF switches. In high VSWR ratios, that condition could occur, for example, when setting up the process recipe in the process chamber and other anomalies in the chamber.

The switching circuit 1100 can further include an intermediary switch M10 for enabling the passive switch 1110 to switch simultaneously with the driving switch M1. In the exemplified embodiment, the intermediary switch M10 is a MOSFET, though in other embodiments other switches can be used. In the exemplified embodiment, the intermediary switch M10 can allow the switching circuit 1100 to disconnect the gate of switch J1 when switch M1 is in the OFF condition. This allows slightly larger high voltage potentials on terminal A.

The exemplified intermediary switch M10 comprises a gate, a drain, and a source. The gate is operably coupled to the power source 1160. In this embodiment, the gate of the intermediary switch M10 is connected to output of the gate driver 1102, though other designs can be used. Further, the drain of the intermediary switch M10 is operably coupled to the passive switch 1110. In the exemplified embodiment, the drain of the intermediary switch M10 is connected to the gate of switch J1.

If the intermediary switch M10 were not in the gate of switch J1, the gate of switch J1 would be connected to the virtual ground or a bottom terminal B. The maximum voltage on the drain of the driving switch M1 would be low in all situations. The drain of driving switch M1 would only see the V_(GS) (gate-source voltage) voltage drop of switch J1. In this case, all the voltage drops from terminal A down to terminal B would be across the passive switch 1110—in the exemplified embodiment, switches J1, J2. Since the driving switch M1 is used only as a switching device at relatively low voltages, the driving switch M1 would likely be an inexpensive and high speed device.

But in the exemplified embodiment, due to the intermediary switch M10, the gate of driving switch M1 and the gate of switch M10 can be switched simultaneously. As a result, the off condition of the series connected driving and passive switches M1, 1110 (e.g., vertically cascoded switches M1, J1, J2) behave differently. Specifically, the high voltage from terminal A down to terminal B can be distributed more evenly. Instead of two transistors taking all the high voltage distribution, there can be three transistors distributing the high voltage—in the exemplified embodiment, switches J1, J2 and M1—since the gate of J1 is opened with the switching action of intermediary switch M10.

In this example, switch M1 should have a high voltage rating similar to those of switches J1, J2. If the gate of switch J1 would be connected directly to ground, the switching circuit could use switch M1 at a relatively low drain voltage. For lower switching voltages, such a configuration could be used. Switch M10 can be a low drain current device with a high drain voltage breakdown that does not have to be heat sunk.

The switching circuit 1100 can further include voltage sense capacitors C1, C2, C3 and C4. A typical design of such a high voltage probe is in the ratio of 1000:1. The capacitors C1, C2, C3 can therefore be in the order of 10 pF and at very high break down voltage. Capacitor C4 can be adjusted accordingly to read 1000:1 in voltage ratio.

Resistors R3, R4 can be included to keep the switches J1, J2 on. Further, these resistors R3, R4 can have very high values (e.g., 10 Mega Ohms or more), and thus can prevent excessive bleed off of RF current in the off condition.

In the exemplified embodiment, transformers T1, T2 can be pulse transformers with a winding isolation in the order of greater than 4 kV. These transformers T1, T2 can detect small voltages in the order of few millivolts to a few volts. Both are used as detector transformers. They can be very small in size but, since they are pulse transformers, they must have very large frequency bandwidth response. The bandwidth is typically determined so that the switching circuit 1100 can detect up to the fifth harmonic of the RF signal being switched. For instance, if the switching circuit was to switch a high voltage and high current waveform at a fundamental frequency of 10 MHz, the detecting transformers T1, T2 can have their bandwidths up to 50 MHz to clearly replicate the drain voltage after capacitive dividers C1, C2, C3 and C4 for the drain voltage and the current transformer T2 that will replicate the source current in switch M1.

FIG. 12 provides a schematic of a second switching circuit 1200. The second switching circuit 1200 is very similar to the first switching circuit 1100 of FIG. 11, and therefore most of the discussion with regard to FIG. 11 applies to also to FIG. 12. Accordingly, comparable components are shown, including gate driver 1202, optocoupler 1206, driving switch M1, voltage sensing circuit 1230, current sensing circuit 1240, monitoring circuit 1250, first comparator 1251, second comparator 1252, AND logic gate 1253, control signal 1254, and power source 1260. Voltage isolation 1270 is also shown.

The primary difference from first switching circuit 1100 is that the second switching circuit 1200 uses MOSFETs M1, M2, M3 in series, instead of a combination of MOSFETs M1 and JFETs J1, J2. Thus the passive switch 1210 uses MOSFETs M2, M3 instead of switches J1, J2. The MOSFETs M1, M2, M3 in the shown vertical position will typically have smaller inter-electrode capacitance. In particular, the MOSFET M3 will typically have a smaller capacitance between the drain and gate and the drain to source than a JFET for the same current. Since the capacitance will be smaller, the second switching circuit 1200 can operate at slightly higher frequencies. The MOSFETs M2, M3 must be kept on (as with switches JFETS J1, J2), but each MOSFET has its own floating DC voltage to keep it in the on condition. Those floating gate-source voltages V_(GS2), V_(GS3) for M2 and M3 are provided by the power supply 1260. To provide the necessary floating voltages for MOSFETs M1, M2, and M3, the power supply 1260 (an SMPS) is designed to have three separate secondary windings from which to make three isolated DC voltages—V_(GS1) (the gate-source voltage of M1), V_(GS2) (the gate-source voltage of M2), and V_(GS3) (the gate-source voltage of M3). The MOSFET M1 is the master switch to turn on and off the whole vertical chain of MOSFETs M1, M2 and M3. In both the first switching circuit 1100 and the second switching circuit 1200, the clock input terminal to the power supply SMPS 1260 can be from 100 kHz to 1 MHz in switch mode supplies. The power supply SMPS also can use the external DC voltage +V_(DD) from an external DC supply. In one embodiment, the external DC voltage can be in the order of +15 VDC. This DC supply can be used in the first switching circuit 1100 and the second switching circuit 1200.

FIG. 13 illustrates parasitic capacitances on a switching circuit 1250. The switching circuit 1250 includes four MOSFET M51, M52, M53, M54 connected vertically in series. In other embodiments, other switches can be used. Switch terminals D and S are floating. First parasitic capacitances C51-C62 are on each of the MOSFETs. Second parasitic capacitances C_(SM1)-C_(SM4) are from the back body of the MOSFETs to the mounting on the heat sink ground GND. Also shown is coupling parasitic capacitances C_(TX1)-C_(TX4) on the gate driver transformers. The parasitic capacitances present possible frequency limitations, and therefore it can be advantageous to keep the parasitic capacitances as low as practically possible. When designing switching circuits, it can be helpful to first predict the parasitic capacitances and from them calculate the maximum frequency of operation for the switching circuit.

FIG. 14 is a graph of a switched waveform 1260. The waveform 1260 is an example waveform for one embodiment of the switching circuit. The frequency is 2 MHz. The RF voltage switched is 3000 V peak to peak. The switching off time is approximately 2 μs. Note the that the switch does not swing the full 3000 V due to inherited capacitance in the circuit. Such computer analysis of switching performance can be performed using P-spice or similar software.

FIG. 15 is a simplified block diagram that provides a basic model for the switching circuits described above. The switching circuit 1280 includes terminals A and B. The outputs are floating. High power voltage and current terminals are electrically isolated from the logic driven switching conditions. Since the terminals A and B are floating, such switches can be connected in parallel to increase the current capability. Such switches can also be connected in series to achieve even higher voltages. In one example, the switch can provide a high voltage isolation 1170 of more than 5 kV.

There are several advantages to the switching circuits discussed herein. First, the switching circuits eliminate disadvantages associated with PIN diodes. PIN diodes require elaborate RF choke designs to support the off condition and DC forward current for turning the PIN diode on. The PIN diodes also require a high DC voltage to back bias the PIN diode to off. Further, the disclosed switching circuits can be simpler to implement. Further, the disclosed switching circuit designs allow for the addition of further switches (e.g., more FETs) to handle even higher voltages when such switches are connected in series. Further, more switches can be connected in parallel to allow for higher current capability of the switch.

FIG. 16 is a schematic representation of a third switching circuit 1300. In this embodiment, the switching circuit 1300 utilizes gallium nitride high-electron mobility transistors (GaN HEMTs). The circuit 1300 structure is similar to that of cascode or vertically connected FETs. By vertical connection of the GaN HEMTs, the breakdown voltage on the total switch structure can be increased.

The switching circuit may be understood as comprising a first switch 1310 and a second switch T35. The first switch 1310 is coupled to a first switch terminal A, and comprises at least one GaN HEMT. In other embodiments, other FETs can be used. In this embodiment, the first switch 1310 comprises four GaN HEMTs T31-T34 connected in series. In other embodiments, other numbers of transistors can be used. Using more transistors will increase the breakdown voltage of the overall switch, thus allowing higher voltage on the switch in the OFF condition.

The second switch T5 is coupled in series with the first switch 1310 and a second switch terminal B. The second switch comprises a GaN HEMT, though other FETs can be used. The second switch is configured to drive the first switch ON and OFF.

The switching circuit 1300 further comprises at least one isolated power source configured to provide isolated power to the first switch and the second switch. In this embodiment, the power sources for the switching circuit 1300 include transformers TX1, TX2 and a DC power supply (not shown).

Second switch T5 is externally driven by a control signal from a gate driver 1302B. In this embodiment, the gate driver 1302B must also be high-voltage isolated from the low-level circuitry. For this purpose, a commercially available gate driver can be used with the GaN device (e.g., the SI8271 GB-IS isolator from Silicon Labs). Such a gate driver can withstand high-voltage isolation, for example, up to 5 kVDC. The gate driver 1302B can receive DC power from a commercially available DC power supply (e.g., PES 1-S5-S9-M from CUI, Inc.). This DC power supply would be isolated and, in this embodiment, can be provided at VDD of the gate driver 1302B. The switching circuit 1300 can be designed such that all transistors from T31 to T35 are turned ON and OFF at precisely the same time to achieve low switching losses.

In the exemplified embodiment, there are isolated gate drivers 1302A on each of the gates of the passive transistors T31 to T34. The gate drivers 1302A include transformers TX1, TX2. In other embodiments, other configurations of power sources can be used to drive the gates. In this embodiment, the gate drivers 1302A are specially designed to minimize cost. According to the exemplified embodiment, the ON/OFF pulses are rectified by diodes D31-D34 and smoothed out by capacitors C31-C34. Resistors RG1-RG4 are static discharge resistors and can have a value, for example, of 10 kohms. Similar to the gate driver 1302B of driving switch T35, the gate drivers 1302A can receive DC power from a DC power supply (not shown). This DC supply must also be isolated to at least 5 kV in the exemplified embodiment. These DC low power supplies are commercially available (e.g., PES 1-S5-S9-M from CUI, Inc.), and therefore are not discussed in detail. When the gate drivers provide the requisite voltage (e.g., 6V) to the gate of the transistor, the transistor is turned on.

For better distribution of voltages in the OFF condition on the GaN HEMTs, resistors R31-R34 (e.g., 10 Mega Ohms) can be included in parallel to each drain to source. Further, it is noted that the switching circuit 1300 (and the other switching circuits discussed herein) can further include a monitoring circuit similar to that discussed above.

One advantage of the disclosed circuit 1300 is that, by using GaN HEMTs, when the switch is fully ON at RF frequencies, RF AC current can flow in the positive and negative directions. The switching circuit can pass full RF current in both directions. The GaN HEMTs also work well at high frequencies. It is noted that GaN HEMTs can also be used in the other switching circuits discussed in this disclosure.

FIG. 17 is a schematic representation of a fourth switching circuit 1400. This switching circuit 1400 is designed to sustain high voltage swings in the OFF condition. In one embodiment, silicon carbide (SiC) FETs having a breakdown voltage of 1200V are utilized for devices M41-M44. Such a switch can swing the AC RF voltage in the OFF condition to at least 4,000V peak to peak.

In the exemplified embodiment, the configuration of FETs M41-M44 is such that, when the switch is in the OFF condition, a full RF voltage of approximately 4 kV peak-to-peak can be applied between terminals A and B and not conduct any RF current through the FETs. When in the OFF condition, during the positive swing on the terminal A, the body diodes D41 and D42 of FETs M41 and M42 will keep the positive voltage on A in the OFF condition. Likewise, when the negative RF voltage swings on terminal A (in the OFF condition), the body diodes D43 and D44 will not allow the RF current conduction to ground. Some leakage current may occur through the bleeder resistors R41-R44 but will be minor.

In the exemplified embodiment, the bleeder resistors R41-R44 are 10 Mega Ohms and are in parallel to all drain to source terminals on the FETs M41-M44. These resistors R41-R44 can keep equal voltage distribution on the drain to source on all devices when OFF. This will also keep all the drain to source voltages symmetrical. It is understood that the values discussed herein are examples and other appropriate values may be used.

The exemplified switching circuit may be understood as comprising a first switch 1410 and a second switch 1420. The first switch can comprise a single transistor M42 or more transistors M41. The one or more transistors M41, M42 are operably coupled in series with a first terminal A. Each of the one or more transistors M41, M42 has a corresponding diode D41, D42. Further, a drain of each of the one or more transistors is operably coupled to a cathode of the corresponding diode.

The second switch 1420 can comprise a single transistor M43 or more transistors M44. The one or more transistors M43, M44 can be operably coupled in series with a second terminal B. Each of the one or more transistors can have a corresponding diode D43, D44. A drain of each of the one or more transistors can be coupled to a cathode of the corresponding diode. Further, a source of the one or more transistors of the first switch 1410 can be operably coupled to a source of the one or more transistors of the second switch 1420. Thus, in the exemplified embodiment, the source of M42 is coupled to the source of M43. A drain of transistor M41 is coupled to terminal A, and a drain of transistor M44 is coupled to terminal B. The transistors of each switch 1410, 1420 can be coupled source-to-drain. Each of the transistors can further include a corresponding resistor R41-R44 operably coupled in parallel to the corresponding diode. In other embodiments, M41 and M44 can be eliminated, or additional transistors can be added, provided the general features of and relationship between the first and second switch (as described in the independent claims) are maintained.

Gate drivers GD1, GD2, GD4 are isolated and can utilize commercially available gate drivers. These gate drivers are typically designed to sustain at least 5 kV peak voltages. These gate drivers also require a floating DC/DC supplies 1430. The floating DC supplies are also isolated. Further, the DC/DC supplies 1430 are also commercially available and thus will not be separately discussed.

The cascoded FETs can be increased to a higher number such as six or more devices connected vertically. In that case the higher RF AC voltage swing in the OFF case will be achieved. For that connection, the gate drivers must sustain higher breakdown voltages. Also, the floating or isolated DC/DC supplies must sustain higher isolation voltages. It is noted that the different potential characteristics of gate drivers and DC power supplies discussed herein can also apply to the gate drivers and DC power supplies of this switching circuit 1400.

The switching circuits 1300 and 1400 have several advantages. The rise time of the ON/OFF condition can be achieved in the order tens of nanoseconds or less. They also enable low capacitance to ground and minimize leakage.

For the circuits discussed above, note that not all components are shown. Rather, the drawings show those components helpful in conveying an understanding of the circuit and its operation. A person of ordinary skill in the art will readily understand the role other standard components can play in the circuit. Further, it is noted that the above switching circuits can be used in methods for providing switching, including methods to provide switching to capacitors or inductors in a matching network, including matching networks in semiconductor fabrication.

While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques. It is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention. Thus, the spirit and scope of the invention should be construed broadly as set forth in the appended claims. 

What is claimed is:
 1. A switching circuit comprising: a first switch comprising one or more transistors operably coupled in series with a first terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; and a second switch comprising one or more transistors operably coupled in series with a second terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; wherein a source of the one or more transistors of the first switch is operably coupled to a source of the one or more transistors of the second switch.
 2. The switching circuit of claim 1 wherein: a drain of the one or more transistors is operably coupled to the first terminal; and a drain of the one or more transistors is operably coupled to the second terminal.
 3. The switching circuit of claim 2 wherein: the one or more transistors of the first switch comprise a plurality of transistors coupled source-to-drain; and the one or more transistors of the second switch comprise a plurality of transistors coupled source-to-drain.
 4. The switching circuit of claim 1 wherein: the one or more transistors of the first switch comprise two transistors; and the one or more transistors of the second switch comprise two transistors.
 5. The switching circuit of claim 1 wherein: the one or more transistors of the first switch comprise three transistors; and the one or more transistors of the second switch comprise three transistors.
 6. The switching circuit of claim 1 wherein each of the transistors of the first switch and the second switch have a corresponding resistor operably coupled in parallel to the corresponding diode.
 7. The switching circuit of claim 1 further comprising: a monitoring circuit configured to: receive an indication that a switching circuit voltage exceeds a predetermined amount and, in response, reduce a power provided to the driving switch; or receive an indication that a switching circuit current exceeds a predetermined amount and, in response, reduce a power provided to the driving switch.
 8. The switching circuit of claim 7 wherein: the indication that the switching circuit voltage has exceeded a predetermined amount is provided by a voltage sensing circuit, the voltage sensing circuit comprising (a) a first transformer operably coupled to the second terminal and (b) a first diode operably coupled to the monitoring circuit; and the indication that the switching circuit current has exceeded a predetermined amount is provided by a current sensing circuit, the current sensing circuit comprising (a) a second transformer operably coupled to the second terminal and (b) a second diode operably coupled to the monitoring circuit.
 9. A method of switching a circuit, the method comprising: providing a first switch comprising one or more transistors operably coupled in series with a first terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; providing a second switch comprising one or more transistors operably coupled in series with a second terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; operably coupling a source of the one or more transistors of the first switch to a source of the one or more transistors of the second switch; driving the first and second switches simultaneously ON; and driving the first and second switches simultaneously OFF.
 10. The method of claim 9 wherein: a drain of the one or more transistors is operably coupled to the first terminal; and a drain of the one or more transistors is operably coupled to the second terminal.
 11. The method of claim 10 wherein: the one or more transistors of the first switch comprise a plurality of transistors coupled source-to-drain; and the one or more transistors of the second switch comprise a plurality of transistors coupled source-to-drain.
 12. The method of claim 9 wherein: the one or more transistors of the first switch comprise two transistors; and the one or more transistors of the second switch comprise two transistors.
 13. The method of claim 9 wherein: the one or more transistors of the first switch comprise three transistors; and the one or more transistors of the second switch comprise three transistors.
 14. The method of claim 9 wherein each of the transistors of the first switch and the second switch have a corresponding resistor operably coupled in parallel to the corresponding diode.
 15. The method of claim 9 further comprising: receiving an indication that a switching circuit voltage exceeds a predetermined amount and, in response, reducing a power provided to the driving switch; or receiving an indication that a switching circuit current exceeds a predetermined amount and, in response, reducing a power provided to the driving switch.
 16. The method of claim 15 wherein: the indication that the switching circuit voltage has exceeded a predetermined amount is provided by a voltage sensing circuit, the voltage sensing circuit comprising (a) a first transformer operably coupled to the second terminal and (b) a first diode operably coupled to the monitoring circuit; and the indication that the switching circuit current has exceeded a predetermined amount is provided by a current sensing circuit, the current sensing circuit comprising (a) a second transformer operably coupled to the second terminal and (b) a second diode operably coupled to the monitoring circuit.
 17. A method of fabricating a semiconductor, the method comprising: placing a substrate in a plasma chamber configured to deposit a material layer onto the substrate or etch a material layer from the substrate; energizing plasma within the plasma chamber by coupling RF power from an RF source into the plasma chamber to perform a deposition or etching; and while energizing the plasma, carrying out an impedance match by an impedance matching network coupled between a load and an RF source, wherein the impedance matching network comprises: a first variable component providing a first variable capacitance or inductance; and a second variable component providing a second variable capacitance or inductance; and wherein each of the first variable component and the second variable component has a plurality of switching circuits configured to provide the first variable capacitance or inductance and the second variable capacitance or inductance, respectively, each of the plurality of switching circuits comprising: a first switch comprising one or more transistors operably coupled in series with a first terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; and a second switch comprising one or more transistors operably coupled in series with a second terminal, wherein each of the one or more transistors has a corresponding diode, a drain of each of the one or more transistors being operably coupled to a cathode of the corresponding diode; wherein a source of the one or more transistors of the first switch is operably coupled to a source of the one or more transistors of the second switch.
 18. The switching circuit of claim 17 wherein: a drain of the one or more transistors is operably coupled to the first terminal; and a drain of the one or more transistors is operably coupled to the second terminal.
 19. The switching circuit of claim 18 wherein: the one or more transistors of the first switch comprise a plurality of transistors coupled source-to-drain; and the one or more transistors of the second switch comprise a plurality of transistors coupled source-to-drain.
 20. The switching circuit of claim 17 wherein: the one or more transistors of the first switch comprise two transistors; and the one or more transistors of the second switch comprise two transistors. 